redundant mail server

can I setup 2 machine running ISPConfig 3.x and both of them can redundant each other? e.g. server A got domain abc.com (IP 1.2.3.4)and server B got 123.com (IP 5.6.7.8) and running as mail server. If server A is down, all abc.com will forward to 5.6.7.8 and temp store in server B, when Server A up again, the mail will send back to Server A. Same when server B is down. I know this also need DNS MX record to help. I already setup MX 10 point to 1.2.3.4 and MX 20 point to 5.6.7.8. the remaining should be how can postfix receive the emails if there's no such domain record? Is there any way I can set in ISPConfig? or I need to amend relay domain in main.cf ?

I saw relay_domains in main.cf point to mysql-virtual_relaydomains.cf, and in that file, it point to a mail_transport in dbispconfig. After all checkings, I know that it's about Email Routing in ispconfig. So, is that means I only need to add a abc.com in Email Routing section?

In Email Routing, there is a destination field, what I have to put in? the 123.com's IP?

then test it something like this:
- add it
- turn off postfix on server a
- "tail -f /var/log/mail.log" on server b
- send mail to [email protected]
- check the log if it's accepted
- turn on postfix on server a
- check logs if it relays
- check mailbox on server a